1. Field of the Invention
This invention relates to a high-transparency, low-emissivity window film or coating. The technology has particular, but not exclusive, application as an energy-saving enhancement for windows in buildings, vehicles, and passive solar heat absorbers.
2. Description of the Related Art
In double-paned glass windows, approximately 60% of the heat transfer through the window center occurs not by conduction or convection, but by the absorption and re-emission of long-wavelength infrared—the so-called blackbody radiation emitted by objects at or near room temperature—with wavelengths between 5 and 20 microns. This occurs because glass strongly absorbs radiant energy at these wavelengths, and thanks to an emissivity of 80-90%, it also strongly emits at these wavelengths when warmed. Thus, although the glass is not transparent to long-wave infrared, through absorption and re-radiation it behaves in some ways as though it were.
Traditionally, this heat transfer is reduced through the addition of a low-emissivity coating to one or more of the glass surfaces. For example, a thin (<10 nm), film of aluminum, silver, or gold has an emissivity of around 10%-25% meaning (a) that the film is highly reflective to long-wavelength infrared, and (b) that when the film is heated, it tends to retain its heat rather than radiating it away. However, these films are still largely transparent, i.e., they allow a large fraction of visible and near-infrared (NIR) light to pass through unattenuated. This sharply reduces the radiative heat transfer across the air gap between the window panes, and thus increase the effective insulation value of the double-paned structure, while still allowing it to serve as a window.
A “dielectric mirror” is a multilayered structure in which the mismatch of dielectric constant between two materials (e.g., a metal and a transparent ceramic or polymer) is maximized. This produces an optical index mismatch that results in very high reflectivity across a broad band of wavelengths, with the corner frequency (i.e., the cutoff between high transparency and high reflection) being determined by the thickness of the layers. In essence, a dielectric mirror is the exact opposite of an antireflection coating. Since the 1990s it has become standard practice to enhance the low-emissivity properties of metal films by assembling them in one or more alternating layers of metal and dielectric to maximize reflection of long-wavelength “blackbody” infrared radiation. This allows for emissivities as low as 2.5% in a structure that nevertheless transmits a significant fraction of visible and solar radiation (e.g., Tvis=41.4% and Tsol=21.5% for AFG Sunbelt Low-E glass as reported in the International Glazing Database, version 15.1).
More recently, pyrolytic ceramic coatings have also seen use as low-emissivity window filters (as described for example in The MSVD Low E ‘Premium Performance’ Myth, John D. Siegel, International Glass Review, Issue 1, 2002). Such coatings are semi-crystalline metal oxides such as tin oxide (SnO) deposited onto the glass by a chemical vapor deposition (CVD) process. These tend to have higher emissivities than metal and metal-dielectric coatings, but are more transmissive to visible and NIR photons. This gives them a higher solar heat gain coefficient (SHGC), making them more suitable for use in cold, sunny climate zones. These coatings are also more robust than “soft” metallic coatings and thus easier to handle in an industrial setting, for example because they do not require a brushless washing process. Indium tin oxide (ITO), a conductive ceramic widely used in video displays, has also seen limited use as a low-emissivity coating, particularly in “heat mirrors” designed to keep optics cool. In Transparent Conducting Oxides, MRS Bulletin August 2000, David S. Ginley and Clark Bright disclose other “low-E” ceramic coatings including cadmium tin oxide (Cd2SnO4), zinc tin oxide (ZnSnO4), magnesium indium oxide (MgIn2O4), gallium indium oxide (GaInO3), zinc indium oxide (Zn2In2O3), copper aluminum oxide (CuAIO2), and aluminum silicon nitride (AlSiN).
Unfortunately, even for very transparent, pyrolytic, low-E coatings, some reflection and absorption of photons also occurs in the coating itself in the visible and particularly in the NIR wavelengths, so that the coated glass pane is only 80-90% transmissive to visible light and 50-65% transmissive to the solar spectrum overall vs. 77% for standard 6 mm clear float glass and tip to 90% for low-iron glass of the same thickness. This reduces the solar heat gain through the pane, which may be undesirable in colder climate zones where solar heating is desirable, or in passive solar devices whose primary purpose is to gather and store solar heat.
Certain optical filters, including distributed Bragg reflectors and Rugate filters, can be designed to operate in blackbody wavelengths, and may seem to behave as ideal longpass filters within a certain limited range. In other words, they reflect radiant energy above a threshold wavelength and transmit radiant energy below that wavelength. However, all of these filters are actually band reflectors, meaning they become transmissive again above a second threshold wavelength. In fact, the practical limitations of real-world materials generally limit the bandwidth of these reflectors to no more than a few hundred nanometers, which is a small fraction of the ˜15,000-nm-wide blackbody spectrum.
Wire grid polarizers attenuate slightly more than 50% of the light passing through them by reflecting away light of one polarization while allowing light of perpendicular polarization to pass through. Wire-grid polarizers that reflect infrared light, rather than absorb it, have been described since the 1960s, for example, in U.S. Pat. No. 4,512,638 to Sriram, et al. With the advent of nanoscale lithography in the 1990s and 2000s, it became possible to produce broadband wire-grid polarizers that reflect in visible wavelengths, for use with high-end optics and laser technology as described, for example in U.S. Pat. No. 6,122,103 to Perkins, et al. Such devices require parallel metal wires (technically a “grating” rather than a “grid”, although the latter term is commonly used) that are tens of nanometers wide and spaced hundreds of nanometers apart. One advantage of these polarizers is that they are just as effective in reflecting/polarizing long-wavelength infrared, and even microwave and radio wavelengths, as they are in polarizing visible light.
Numerous conductive meshes, grids, and drilled plates that are used as shields against, or reflectors of, microwave and radio frequency (RF) radiation are known. One example is the metal screen covering the glass or transparent plastic door of a microwave oven. As long as the holes in the mesh are significantly smaller than the 5-12 cm wavelength output by the oven's magnetron, the mesh will behave to those wavelengths as though it were a uniformly conductive film or plate, and thus will reflect the microwave radiation. However, because visible light has a wavelength much smaller than the holes in the mesh, it is able to pass through the holes just as it would through holes in a non-conductive material. Thus, food cooking inside the oven is visible without exposing the operation to harmful microwave radiation.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.